Keywords:Chauchou fault, noble gas, active tectonics, mantle-derived fluids
Here we use helium isotopes in groundwater from along the Chauchou/Tulungwan fault system in southern Taiwan to evaluate the involvement of the upper mantle in an active arc-continental collision. Helium isotope ratios of sixteen groundwater samples, four bedrock samples, and one hot spring sample, collected along 10 km on the fault, were measured and reported as RA (relative to an air helium isotope ratio (3He/4He) of 1.39 × 10-6). Measured groundwater helium isotope ratios, ranging from 0.07 to 1.00 RA, express a clear mixing model with three endmembers of air (1.00 RA), crust (0.06 RA) and upper mantle (8 RA). Samples from southern part of the fault show normal distribution of crustal signal. By correcting the helium from air-contamination, samples from northern part of the fault with 0.30-0.78 RA reveal significant upper mantle signal. The Chauchou/Tulungwan fault system, in the area of detected mantle-derived fluids, projects down dip to a zone of ambient tremors and a nearby zone of high conductivity; both features extend to nearly 40 km, which is close to the crust-mantle boundary in this area. These observations suggest that mantle-derived fluids penetrate the crust through the zone of the tremor activity, reaching the surface long the Tulugwan fault zone. This study shows that non-volcanic, mantle-derived fluids can be involved in tectonic processes associated with an active arc-continent collision zone.